This invention relates to a method and apparatus for maintaining a clean printing surface on a blanket cylinder of a printing press such as a web offset press, or the impression cylinder of a letter press or a gravure press, during the printing operation. The blanket or impression cylinders of these presses tend to rapidly accumulate foreign material such as lint, dust and ink on their peripheral surface portions, which negatively affects the quality of the printed product. The cylinders must, therefore, be cleaned in order to remove this foreign material. One way of cleaning these cylinders has been to periodically discontinue printing and manually wash off the rotating blanket cylinders with a solvent soaked rag. This operation can be extremely dangerous, and it is additionally undesirable because of the press shutdown time required.
Other methods and apparatuses for cleaning blanket cylinders are disclosed in Switall U.S. Pat. No. 3,508,711 and Anderson et al. U.S. Pat. No. 3,486,448. In accordance with these patents, the cleaning is accomplished by spraying solvent onto the blanket cylinders and onto the paper web while the press is running. The foreign material is thus washed from the blanket cylinder surface and adheres to the paper web which is moving through the press. While this type of an arrangement is a significant improvement over manual washing of the blanket cylinder, it still requires the utilization of volatile solvents, and the actual printing operation must be interrupted during the cleaning operation.
Another method for cleaning blanket cylinders is disclosed in U.S. Pat. Nos. 4,875,412 and 4,667,597 to Wright, et al. The cleaner disclosed in these patents incorporates a non-rotating brush periodically urged against the blanket cylinder. Disposed beneath the brush is a lint catcher. As the brush is retracted, a hinged door on the lint catcher opens to receive dust and lint loosened by the brush. The specification states that fluid or vacuum devices may be employed to provide for automatic removal of lint and debris from the lint catcher. The specification states that the invention may be used during the uninterrupted course of a press run. However, in practice, printers using the commercial embodiment of the disclosed device prefer to cycle the cleaning operation during the time period when a splice between two paper webs occur. This is because the operation of the cleaning device produces a noticeable affect in the printing, requiring the printed material to be rejected. However, the material printed during the splice is also rejected, so one rejected section of the web will contain the results of both the splice and the cleaning operation.
One disadvantage of cleaning only on the splice is that lint buildup may be excessive before the splice occurs. Also, if more frequent cleanings could be made, the overall printing quality would be improved as less lint would build up between cleanings.
Another disadvantage of the commercial embodiment of the Wright, et al. device is that the printing tends to be smeared during the cleaning cycle. This is due to the fact that a portion of the ink transferred to the blanket remains on the blanket after contacting the web. Usually this does not cause a problem because the new print image and the leftover image are always synchronized as a result of adherence to strict tolerances on the diameter of the blanket cylinder and plate cylinder. However, when the brush of the commercial Wright, et al. device contacts the blanket, the residual ink is smeared slightly. Thus, on the next revolution, the fresh ink and residual ink are not perfectly aligned, resulting in what is known as a "slurred image" print.
Another disadvantage of the commercial device is that the brushes tend to cause the blanket on the cylinder to bunch up and form a hump at the trailing edge where the blanket is secured into the blanket cylinder. This is caused, in part, because the force used to hold the brush in a cleaning position all bears on the blanket. The result is excessive wear at the hump, and reduced blanket life, as well as poor quality printing during the cleaning cycle.
Another drawback to prior art cleaning devices is that most blanket cylinders are mounted on the press frame so that they can print in either of two positions (sometimes referred to as a "black" position and a "three color" position). Also, the blanket cylinder may be moved to an intermediate position (sometimes referred to as an "off" position). In this position, the blanket cylinder is not in contact with the plate cylinder or the other blanket cylinder (used for two-sided printing). This prevents flattening of the blanket cylinder surface which would otherwise occur. It would be preferable to be able to use a single cleaning apparatus to clean the blanket cylinder in all of its possible positions, without using a complicated mounting systems in the crowded area available on the press frame.